The dependence on cloudiness and turbidity of the monthly average daily diffuse-global correlation

Abstract

Correlations between HdH and HHo, HdH and HHo have been established using data obtained in the period 1951–1980 at Uccle (Belgium), Hd being the monthly average daily diffuse radiation, HD the monthly average daily direct radiation, H the monthly average daily global radiation, and Ho the monthly average daily extraterrestrial radiation, all of them on a horizontal surface. The correlations have been obtained for different sky conditions A, B, C. (A) Clear skies (fraction of possible sunshine = 1) and minimum turbidity, corresponding to the maximum values of radiation measured during the period considered; (B) clear skies and average turbidity, corresponding to all the values of radiation measured when the fraction of possible sunshine is 1 during the period considered; (C) any sky conditions, corresponding to all the solar radiation data collected during the period considered. Linear correlations obtained for data corresponding to condition B and 0.676 >HHo > 0.573 and for condition C and 0.437 >HHo > 0.254 are very similar to those obtained when both sets of data are used together. The following linear correlations obtained using both sets of data: HdH = 0.960–0.983HHo and HdH = 0.015+0.950HHo can be used with high accuracy for conditions B or C. When data obtained for condition A are used no linear correlations are obtained.